We first exploit the available basis degrees of freedom to recast the model in the form most suitable for making analytical progress
in deriving the general BCs and subsequent solution of the bound-state problem.
(We stress that this is always possible for the model of the most general form; no specific assumptions about its form are made.)
Such degrees of freedom are:
(i) the SU(2) rotation (three real parameters) of the wave-function basis;
(ii) the coordinate and momentum basis; the latter becomes particularly important in higher dimensions.
Such procedure has already been carried out in the 2D case in Ref. [1]}.
